1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a facsimile, a printer, and a like for forming an image by using an electronic photograph method and an inkjet method, and more particularly to an adjustment method for adjusting a plurality of reading parts, an image reading device for adjusting a plurality of reading parts, and an image forming apparatus for adjusting a plurality of reading parts, in which higher image quality of an image can be realized in that no image displacement caused by duplicated pixels and dropped pixels occurs.
2. Description of the Related Art
In the following, one of technologies concerning an image reading device will be described with reference to FIG. 1 through FIG. 3. FIG. 1 is a schematic diagram showing a configuration of the image reading device. The image reading device 10a recognizes that a sheet 11 is inserted into the image reading device 10a, when a first paper detector 9 detects an edge of the sheet 11 inserted on the sheet table 8. Then, the image reading device 10a causes a first carriage roller 1 and a second carriage roller 2 to rotate, and determines a start timing to read the sheet by using a second paper detector 10. When the sheet 11 is lead by the first carriage roller being rotated to a read sensor 3, the sheet 11 is read by the read sensor 3. When a reading process for the sheet is completed, the sheet 11 is ejected outside the image reading device 10a by the second carriage roller 2.
A lighting unit 5 is provided inside the read sensor 3. Light having a predetermined light quantity is illuminated with respect to an image surface of the sheet 11, Reflected light, which is reflected from the image surface and corresponds to an image pattern, is focused on a photodetector 7 to form an image at an actual magnification through a SELFOC lens. An analog output level of the photodetector 7, which corresponds to an image of the sheet 11, is converted into a digital value from an analog by an A/D conversion circuit, and conversion data are accumulated in a memory as image data.
FIG. 2A is a diagram for explaining a first type of the read sensor. FIG. 2B is a diagram for explaining a second type of the read sensor. FIG. 2C is a diagram for explaining a third type of the read sensor. Reading methods are broadly classified into three types. In a first type of the read sensor shown in FIG. 2A, the sheet 11 is read out by using a read sensor 3-a, of which a shape is a single bar corresponding to a maximum width of the sheet 11. In a second type of the read sensor shown in FIG. 2B, a plurality of segmented sensors 3-b are arranged in a width direction of the sheet 11, and images read by the plurality of segmented sensors 3-b are combined (for example, see Japanese Laid-open Patent Application No. 59-105762). In a third type of the read sensor shown in FIG. 2C, the reading method is similar to the second type of the read sensor in FIG. 2B in that the sheet 11 is read by a plurality of segmented sensors 3-c. However, in the third type, a reduction lens 12 is used for each read sensor 3-c being a reduced type.
In the first type, an image having a high quality can be obtained by a simple configuration. However, the single read sensor 3-a is required to read the sheet 11 at any size of the sheet 11 in the width direction. A cost of a component of the read sensor 3-a is proportional to a length of the sheet 11 in the width direction. For example, in a case of reading the sheet 11 having an A0 width, a sensor having an A0 length is needed, and the cost of the component of the read sensor 3-a is increased. As a result, a cost of the image reading device 10a is increased. This is a problem of the first type.
The second type improves the first type. In the second type, the plurality of read sensors 3-b having a shorter width than the width of the single read sensor 3-a are arranged, so as to reduce the cost of the component of the read sensors 3-b. However, the second type is required to combine images read by the plurality of read sensors 3-b in order to form a composite image. As a result, an image data process becomes complicate more than the image data process in the first type.
In the third type, the cost of the components of the read sensors 3-c can be reduced more than the cost of the single read sensor 3-a, similar to the second type. In order to further reduce the cost more than the second type, the read sensors 3-c being the reduced type are used.
In an image reading method in the second type in detail, two of the read sensors 3-b are arranged at an upstream side in a sheet carrying direction, and read out two respective parts of images of the sheet 11 along a width direction of sheet 11. Subsequently, one of the read sensors 3-b is arranged at a downstream side in the sheet carrying direction, and reads out a respective part of the image along the width direction of sheet 11, which is not read out by two of the read sensors 3-b arranged at the upstream. In this case, image data, which are read out by the read sensor 3-b arranged at the upstream, are temporarily stored in a predetermined memory to cause a delay, and are combined with image data read by the read sensor 3-b arranged at the downstream.
As described above, in the second type, partial images read by the plurality of the read sensors 3-b are required to be combined. In order to normally join the partial images by combining the partial images with each other, it is required to ensure a location accuracy of the read sensors 3-b, and to precisely define a start pixel and an end pixel to read out the image in a main scanning direction of the read sensors 3-b arranged at the upstream and the downstream, simultaneously. Moreover, a relative distance between the read sensors 3-b at the upstream and the downstream is calculated in a sub-scanning direction. The image data read by the read sensor 3-b at the upstream are delayed by the relative distance. A correction operation is required to combine the image data read by the read sensors 3-b at the downstream with the image data read by the read sensor 3-b at the upstream on a single line.
FIG. 3A, FIG. 3B, and FIG. 3C are diagrams showing a case in that an image displacement is caused by a setting displacement between the start pixel and the end pixel to read out the image at a read joint of the read sensors 3-b. In FIG. 3A through FIG. 3C, a distance K is an interval between intersection points in the main scanning direction of a diamond-shaped image.
FIG. 3B is a diagram showing the image displacement in a case of setting the end pixel of a first read sensor at the upstream side to be a pixel N when a pixel 1′ is defined as the start pixel of a second read sensor at the downstream. In a case of adding two pixels (+2 pixels) to a proper end pixel N−2 of the first read sensor at the upstream side in order to set the end pixel of the first read sensor at the upstream side at a pixel N, the number of pixels for an intersection interval in the main scanning direction of the diamond-shaped image partially overlapping joints becomes K+2 pixels. Since the diamond-shaped image partially overlaps two pixels, the diamond-shaped image is extended in the main scanning direction. As a result, the diamond-shaped image is displaced in the sheet carrying direction at the joints.
FIG. 3C is a diagram showing the image displacement in a case of setting the end pixel of the second read sensor at the downstream side to be a pixel N−4 when the pixel 1′ is defined as the end pixel of the first read sensor at the upstream. In a case of deducting two pixels (−2 pixels) from the proper end pixel N−2 of the first read sensor at the upstream side in order to set the end pixel of the first read sensor at a pixel N−4, the number of pixels for an intersection interval in the main scanning direction of the diamond-shaped image partially overlapping joints becomes K−2 pixels. Since two pixels are dropped in the diamond-shaped image, the diamond-shaped image is reduced in the main scanning direction. As a result, the diamond-shaped image is displaced in the sheet carrying direction at the joints.
There are other methods for operating correction to compose image data with each other in which the start pixel and the end pixel in the main scanning direction to read the sheet 11 are set, as follows:    (1) method in which each relative distance in the main scanning direction and the sub-scanning direction at joints of the read sensors arranged at the upstream and the downstream is mechanically measured, each relative distance is converted into a correction value, the start pixel and the end pixel to read image data in the main scanning direction are set, and a delay amount in the sub-scanning direction of the image data read by the read sensor at the upstream is set.    (2) method in which a monitor is connected to an image reading device being the second type in order to display a read image, for example, as shown in FIG. 3A, FIG. 3B, and FIG. 3C, the diamond-shaped image and one straight line orthogonal to the sub-scanning direction are read and displayed at the monitor, the start pixel and the end pixel to read the image data in the main scanning direction are set while displaying a displacement state at each joint of the read sensors arranged at the upstream and the downstream of a displayed image, each relative displacement in the sub-scanning direction is measured and converted into the correction value, and the delay amount is set for the image data read by the read sensor arranged at the upstream.    (3) method in which one straight line orthogonal to the sub-scanning direction is read by each of the read sensors, each regression line is automatically obtained by a control circuit configured by a CPU and a like based on pixel data of a line image, a relative displacement amount in the sub-scanning direction is obtained at each joint of the read sensors, and the delay amount of the image data is set (for example, see Japanese Laid-open Patent Application No. 8-97980).
However, the methods described in the above items (1) and (2) take time and workload to measure the displacement amount at the joints of the read sensors arranged at the upstream and the downstream, and increase a production cost. Moreover, deviation of measurement results by various operators occurs, and a precise setting cannot be conducted. As a result, as shown in FIG. 3A through FIG. 3C, an image displacement is caused to the read image, and the read image becomes different from an original image.
In the method described in the above item (3), since the CPU and the like automatically measure one straight line in the sub-scanning direction and set the correction value, problems in the methods described in the above item (1) and (2) are reduced. In this method, there is no problem when a distance between the read sensors at the upstream and the downstream is a slight distance. However, in general, a width of each reading sensor in the sub-scanning direction is required to be sufficient for a width of a substrate mounting and operating a photodetector and a width of a chassis. In a case in that a staggered arrangement is conducted to the read sensors, a distance between the read sensors arranged at the upstream and the downstream becomes larger. When a displacement of a sheet carrying speed occurs and cannot be ignored during the distance, an image is read at a relatively different speed from an expected sheet carrying speed. On the other hand, the delay amount is defined based on this image. As a result, the image displacement usually occurs to most of a composite image.